Method for depositing a fluorine-doped silica film

ABSTRACT

The invention concerns a method which consists in evaporating silicon oxide to form a silicon oxide film at the surface of a substrate and in bombarding said silicon film, while it is being formed, with a beam of positive ions derived from both a polyfluorocarbon compound and a rare gas. The invention is useful for producing low-index antiglare films.

[0001] The invention relates in a general manner to a process fordepositing a fluorine-doped silica film (SiO_(x)F_(y)) on a surface of asubstrate, in particular of an ophthalmic lens.

[0002] Silica-based (SiO₂) thin films are widely used in optics and moreparticularly in the field of ophthalmic optics. Such silica-based thinfilms are used in particular in anti-glare coatings. These anti-glarecoatings are conventionally constituted of the multi-layered stacking ofinorganic materials. These multi-layered anti-glare stackings usuallycomprise one or more layer(s) having a low refractive index in thevisible spectral field. Conventionally, these layers of low refractiveindex are constituted of a silica-based thin film.

[0003] The deposition techniques for such silica-based thin films arevery diverse, but deposition by evaporation under vacuum is one of themost commonly used techniques. These SiO₂-based thin films possess verysatisfactory mechanical properties and refractive indices usually of theorder of 1.48 for a wavelength around 630 nm.

[0004] However, in order to be able to improve the optical performancesof the anti-glare stacking on the one hand and to generate novel systemsof anti-glare stacking, it would be desirable to be able to lower therefractive index of this low index film while preserving itssatisfactory mechanical properties.

[0005] In order to resolve this technical problem, it has already beenproposed to generate porous silica (SiO₂) films, i.e. in which air isimprisoned.

[0006] Unfortunately, as well as the complex manufacturing techniquesemployed, the films thus obtained possess unsatisfactory mechanicalproperties which are inferior to those of a conventional silica thinfilm.

[0007] Moreover, the use of fluorine-doped silica thin films is known inother technical fields, in particular in the field of microelectronics.

[0008] The films are obtained by chemical deposition in the vapour phaseassisted by plasma on discs for semi-conductors.

[0009] This technique induces a heating of the substrate which isbrought to high temperatures, incompatible with the treatment ofophthalmic organic glasses.

[0010] Furthermore, these layers pose stability problems. The patentapplication EP-0.957.017 gives an account of diffusion problems offluorine to the outside of the fluorine-doped silica film which leads toadhesion problems.

[0011] The deposition of a silica film has been proposed in order toprevent this diffusion without, however, giving complete satisfaction.

[0012] The article “Characteristics of SiO_(x)F_(y) Thin Films Preparedby Ion Beam Assisted Deposition” by F. J. Lee and C. K. Hwangbodescribes thin films of fluorine-doped silicon oxide (SiO_(x)F_(y)). Thearticle describes in particular the deposition of thin films ofSiO_(x)F_(y) of a thickness of about 600 nm on glass and silicasubstrates. The basic vacuum pressure is 1.2×10⁻⁴ Pa and the temperatureof the substrate is about 150° C. The silica is evaporated by means ofan electron beam in the presence of oxygen in the chamber and thesilicon oxide deposit is bombarded during its formation by a beam ofpolyfluorocarbonated ions formed by means of an ion gun starting fromCF₄ gas.

[0013] The thin SiO_(x)F_(y) films obtained have refractive indicesvarying from 1.394 to 1.462 and can be used as optical films.

[0014] However, the Si_(x)F_(y) films obtained by the process of theabove article have the disadvantage of taking up water with time and ofhaving an unstable refractive index which increases with time.

[0015] The object of the present invention is thus a process for thedeposition on a surface of a substrate of a fluorine-doped silica film(Si_(x)F_(y)) with a low refractive index, stable over time and havingmechanical properties at least comparable to the films of the prior art.

[0016] According to the invention, the process for the deposition on asurface of a substrate of a fluorine-doped silica film (Si_(x)F_(y))comprises:

[0017] a) the evaporation of silicon and/or silicon oxide;

[0018] b) the deposition of silicon and/or silicon oxide evaporated atthe surface of the substrate in order to form on the said substratesurface a silicon oxide film; and

[0019] c) the bombardment, during its formation, of the silicon oxidefilm with a beam of positive ions derived from a polyfluorocarbonatedcompound or a mixture of polyfluorocarbonated compounds, the processbeing characterized in that the silicon oxide film is also bombarded,during its formation, by a beam of positive ions derived from a rare gasor a mixture of rare gases.

[0020] As indicated above, the deposit of silicon oxide during step b)of the process of the invention is obtained by evaporating siliconand/or a silicon oxide.

[0021] A silicon oxide of formula SiOx with x<2 or SiO₂ may be used.When SiOx with x<2 is used, it is necessary that the ambient mediumcontains oxygen O₂.

[0022] Of course, a SiOx/SiO₂ mixture may be used. SiO₂ silica ispreferred in the framework of the invention.

[0023] The polyfluorocarbonated compound may be a linear, branched orcyclic perfluorocarbonated compound, and is preferably linear or cyclic.

[0024] Among the linear perfluorocarbonated compounds, mention may bemade of CF₄, C₂F₆, C₃F₈ and C₄F₁₀; among the cyclic perfluorocarbonatedcompounds, mention may be made of C₃F₆ and C₄F₈; the preferred linearperfluorocarbonated compound is CF₄ and the preferred cyclic compoundC₄F₈.

[0025] A mixture of perfluorocarbonated compounds may also be used.

[0026] The polyfluorocarbonated compound may also be ahydrogenofluorocarbon,

[0027] preferably selected from CHF₃, CH₂F₂, C₂F₄H₂. Thehydrogenofluorocarbon may also be linear, branched or cyclic.

[0028] Naturally, a mixture of perfluorocarbonated andhydrogenofluorocarbon compounds may be used.

[0029] The rare gas is preferably selected from xenon, krypton and theirmixtures. The preferred rare gas is xenon.

[0030] During the deposition of the fluorine-doped silica layer, thesubstrate is usually at temperature lower than 150° C., preferably lowerthan or equal to 120° C. and better still varies from 30° C. to 100° C.

[0031] In a preferred embodiment of the invention, the temperature ofthe substrate varies from 50 to 90° C.

[0032] The fact that the deposit according to the invention can beperformed at a relatively low temperature makes it possible to form thinfilms on a large variety of substrates and in particular substrates madeof organic glass, such as ophthalmic lenses made of organic glass.

[0033] Usually, the process of the invention is carried out in a vacuumchamber at a pressure of 10⁻² to 10⁻³ Pa. If necessary, oxygen gas canbe introduced into the vacuum chamber during the deposition of the film.

[0034] The fluorine-doped silicon oxide films of the invention usuallyhave a thickness of 10 to 500 nm, preferably from 80 to 200 nm, and theatomic fluorine content of the films is usually from 6 to 10%.

[0035] The silicone content is usually of the order of 30% atomic.

[0036] The fluorine-doped silicon oxide films obtained by the process ofthe invention have a refractive index n≦1.48, and preferably from 1.42to 1.45 (for radiation of wavelength λ=632.8 nm at 25° C.).

[0037] The remainder of the description refers to the appended figureswhich represent respectively:

[0038]FIG. 1, a schematic view of an appliance for carrying out theprocess of the invention; and

[0039]FIG. 2, a schematic plan view of the appliance of FIG. 1.

[0040] The device for depositing thin films shown in the FIGS. 1 and 2assisted by an ion beam is a standard device. This device comprises avacuum chamber, the first extremity 2 of which is connected to one ormore vacuum pumps and the other opposite extremity comprises a door 3. Acold trap 4 can be placed in the chamber close to the extremity 2connected to the vacuum pumps. Within chamber 1 is located an electrongun 5 comprising a crucible 6 designed to contain the silica to bevaporised. The substrates A to be coated are arranged on a support closeto a quartz micro-balance 9. If need be, provision may also be made foran oxygen gas supply to chamber 10. The pressure in the chamber can bemeasured by means of a hot cathode pressure gauge 8. The supply line 11of the ion gun 7 is connected to three feed drive devices for gasesmaking it possible to simultaneously or independently supply the ion gunwith gases of the desired nature and/or flow rates.

[0041] In the present case, the vacuum chamber is a Leybold Heraeuschamber capable of attaining a basic vacuum of 5×10⁻⁵ Pa, the ion gun isMARK II Commonwealth gun, and the electron gun is a Leybold ESV gun.

[0042] For the control feed device of gases of the ion gun, a BROOKSmass flow control device is used for argon gas, itself controlled by theMARK II control device. For the feed of xenon and thepolyfluorocarbonated compound, mass flow control devices are used suchas the multigas control device MKS 647 B in which the nature and flowrate of the gases can be programmed.

[0043] The deposition on the substrates of the fluorine-doped silicafilm according to the invention can be carried out in the followingmanner:

[0044] The chamber 1 is placed under a vacuum of 2×10⁻³ Pa (measured bymeans of the hot cathode pressure gauge 8). The ion gun 7 is primed withargon gas, then CF₄ gas and xenon are introduced at selected flow ratesand the argon flux is interrupted. The silica particles (SiO₂) placed inthe crucible 6 are preheated by the electron beam gun. When oxygen gasis used, it is introduced in the chamber at a fixed flow rate.Simultaneously, the electron beam gun and the ion gun are equipped withan obturator, and the two obturators of the electron beam gun and theion gun are opened simultaneously. The thickness of the deposit isregulated by the quartz micro-balance 9 near to the sample substrates.When the desired thickness of the films is obtained, the two obturatorsare closed, the electron beam and ion guns are cut, the supply of thevarious gases is stopped, and the vacuum of the chamber is broken. Thesample substrates coated with the fluorine-doped silica film accordingto the invention are then recovered.

[0045] The following examples illustrate the present invention.

[0046] By operating as previously described, flat surface siliconsamples have been coated with fluorine-doped silica films. Therefractive index at the wavelength λ=632.8 nm and at 25° C. of thefluorine-doped silica films formed was measured at different times afterthe formation of the films. The absorption of water by the films formedat different times after the preparation of the films was alsodetermined by infrared spectrometry, this absorption beingcharacteristic change of the film with time. The conditions fordepositing the fluorine-doped silica films are indicated in Table 1,while the properties of the films obtained, in particular the refractiveindex and the detection of the presence of water by infraredspectrometry and the thickness of the layers obtained, are indicate inTable 11. TABLE I Deposition conditions Polyfluoro- carbonatedDeposition Ion gun Ion gun Polyfluoro- compound Chamber Substrate rateanode anode carbonated flow rate Xe flow rate O₂ flow rate pressuretemperature Example N° (nm/s) current (A) voltage (V) compound(cm³/minute) (cm³/minute) (cm³/minute) (Pa)⁽¹⁾ (° C.) Comparative 0.510.53 160 CF₄ 2.3 — —   4.10⁻³ 70° C.⁽²⁾ C1 Comparative 0.18 0.3 100 CF₄1.8 — — 5,3.10⁻³ 180° C.⁽³⁾  C2 1 0.75 4 150 CF₄ 2.5 2.9 4 1,8.10⁻² 70°C.⁽²⁾ 2 0.75 0.5 100 CF₄ 1.5 0.5 4 7,9.10⁻³ 70° C.⁽²⁾ 3 0.5 4 150 C₄F₈ 12.7 15 2,4.10⁻² 70° C.⁽²⁾

[0047] TABLE II Properties of the SiOxFy films Refractive index at λ =632.8 nm Presence of water (IR) Example Thickness After 1 After 24 After2 After 2 After 2 After 1 After 2 After 2 N° (nm) hour hours days weeksmonths hour days weeks Comparative 125 1,415 — 1,465 — — No Yes — C1Comparative 110 1,400 1,448 — — — — — — C2 1 190 1,429 — — 1,432 1,435No No No 2 180 1,444 — — 1,449 1,450 No No No 3 190 1,434 — — 1,437 — NoNo No

[0048] The results of Table II show that the bombardment with an ionbeam derived simultaneously from a polyfluorocarbonated compound and arare gas, in this case xenon, makes it possible to obtain a particularlynoteworthy stabilization of the refractive index with time. In fact, therefractive index of the films of the comparative examples C1, C2increased by 3.5% after two days and by 3.4% after 24 hours,respectively, whereas the refractive index of the films of the examples1 to 3 obtained by the process of the invention show only an increase ofless than 0.35% after two weeks, and less than 0.42% after 2 months.

1. Process for depositing on a surface of a substrate a fluorine-doped silica film (Si_(x)F_(y) ) comprising: (a) the evaporation of silicon and/or silicon oxide; (b) the deposition of silicon and/or silicon oxide evaporated on the surface of the substrate in order to form on the said substrate surface a silicon oxide film; and (c) the bombardment of the silicon oxide film, during its formation, by a beam of positive ions derived from a polyfluorocarbonated compound or of a mixture of polyfluorocarbonated compounds; characterized in that the silicon oxide film is also bombarded, during its formation, by a beam of positive ions derived from a rare gas or a mixture of rare gases.
 2. Process according to claim 1, characterized in that the polyfluorocarbonated compound is a linear or cyclic perfluorocarbonated compound.
 3. Process according to claim 2, characterized in that the linear perfluorocarbonated compound is selected from the compounds CF₄, C₂F₆, C₃F₈ and the cyclic perfluorocarbonated compounds are selected from the compounds C₃F₆ and C₄F₈, and preferably C₄F₈.
 4. Process according to claim 1, characterized in that the polyfluorocarbonated compound is a hydrogenofluorocarbon.
 5. Process according to claim 4, characterized in that the hydrogenofluorocarbon is selected from CHF₃, CH₂F₂, C₂F₄H₂.
 6. Process according to any one of the preceding claims, characterized in that the rare gas is selected from xenon and krypton, and is preferably xenon.
 7. Process according to any one of the preceding claims, characterized in that during the deposition of the silicon oxide and bombardment, the substrate is at a temperature lower than 150° C., and preferably lower than 120° C.
 8. Process according to claim 5, characterized in that the substrate is at a temperature between 30° C. and 100° C., and preferably between 50° C. and 90° C.
 9. Process according to any one of the preceding claims, characterized in that it is carried out in a vacuum chamber at a pressure of 10⁻² to 10⁻³ Pa.
 10. Process according to any one of the preceding claims, characterized in that oxygen gas is introduced in to the chamber during the deposition.
 11. Process according to any one of the preceding claims, characterized in that the fluorine-doped silicon oxide film formed has a thickness of 10 to 500 nm, and preferably of 80 to 200 nm.
 12. Process according to any one of the preceding claims, characterized in that the atomic fluorine content of the fluorine-doped silicon oxide film is from 6% to 10%.
 13. Process according to any one of the preceding claims, characterized in that the silicon oxide film has a refractive index, n, at a wavelength of 632.8 nm and at 25° C. of less than 1.48 and preferably 1.42 to 1.45.
 14. Process according to any one of the preceding claims characterized in that the substrate is an ophthalmic lens.
 15. Process according to any one of the claims 1 to 13, characterized in that the substrate is a flat silicon sample. 